Solar radiation budget in the atmosphere under broken cloudy sky. An analytical model. (August 2021)

• Record Type: Journal Article
• Authors: Rosa, Rui Namorado; Silva, Ana Maria
• Is Part Of: Journal of atmospheric and solar-terrestrial physics. Volume 219(2021)
• Publication Date: 2021-08
• Subjects: Broken clouds -- Cloud/sunshine fraction -- Cloud radiative forcing; Geophysics -- Periodicals; Atmospheric physics -- Periodicals; Géophysique -- Périodiques; Météorologie physique -- Périodiques; Electronic journals; 551.51
• DOI: 10.1016/j.jastp.2021.105643
• Languages: English
• ISSNs: 1364-6826

Abstract

Abstract: This study theoretically investigates the transport of broadband solar radiation in the Earth's cloudy atmosphere (extended and broken) by considering three plane parallel layers: i) thin bottom layer adjacent to the Earth's surface; ii) intermediate layer where clouds are most embedded; and iii) upper layer that is essentially cloud-free with possible occurrence of only thin clouds. These three layers are coupled by upwelling and downwelling radiation exchanges, including multiple retro-diffusion, and both direct beam and diffuse components are considered. The interrelation and relative magnitude of the solar radiation flux cascade across the atmosphere - α P, R a, R c, R g and G/H – was established and explored as a tool for the present simulations. Two distinct radiation transport regimes through the inter-cloud gaps are identified, with the regime transition occurring when (AR *TAN(θ )+1)*CF = 1 where AR, θ and CF are the cloud aspect ratio, solar zenith angle, and cloud fraction, respectively. The variation of the solar radiation fluxes with CF and θ exhibit the transition between the two radiation transport regimes, as evidenced in the flux absorbed in the cloudy layer. The effect of AR upon the solar fluxes also exhibits the two radiation transport regimes, with one regime having linear dependence of the various fluxes on AR and the other one having fluxes stay at constant levels independent of AR. The shape factor SF of cloud top and bottom boundary "surfaces" is introduced to account for the self-irradiation effect for different cloud types. It is found that increased roughness of one or both boundaries increases the cloud absorptance in all cases whereas transmittance and reflectance for upwelling and downwelling can either be diminished or enhanced. With the proposed model the estimated impact of clouds on the enhancement of the shortwave radiation absorption in the atmospheric column was improved leading to results closer to the cloudy-sky absorptance observations reported in the literature. The ratio of the cloud radiative forcing at the surface to the cloud radiative forcing at the top of the atmosphere (R net ), obtained by means of the present model, lead to R net = 1.3, closer to the estimate of 1.5 by Cess et al. than the values reported by other cloud radiative transfer models. Highlights: Cloud shape and roughness influence on cloud layer radiation properties and fluxes. Assessment of relationship of surface cloudiness and satellite observed cloudiness. Analytical simulation of solar radiation fluxes across a broken cloud atmosphere. Analytical simulation of cloud solar radiation forcing in a broken cloud atmosphere.